Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

Histamine via histamine H1 receptor enhances the muscarinic receptor-induced calcium response to acetylcholine in an enterochromaffin cell model

As a prerequisite for serotonin secretion, the P‐STS ileal enterochromaffin cell line responds to acetylcholine (ACh) stimulation with an increase in intracellular calcium mediated by the muscarinic ACh receptor M3 (M3R). Histamine increases intracellular calcium via histamine H1 receptor (H1R) in P‐STS cells and pre‐incubation with histamine specifically augments the response to ACh but not to epinephrine or nicotine. We aimed to elucidate whether histamine receptors are involved in this synergism. Astonishingly, HEK‐293 T cells—known to express M3R, but only a very low amount of histamine receptor messenger RNA—showed a similar enhancement of the calcium response to ACh by pre‐incubation with histamine. Despite the much lower level of H1R protein detected in HEK‐293 T cells as compared to P‐STS cells, in both cell lines pre‐treatment with H1R antagonists inhibited the synergism between histamine and ACh. No indication for an involvement of histamine H2 or H4 receptors in the synergism was found. Furthermore, pre‐incubation with the cAMP‐inducing compound forskolin had no influence on the intracellular calcium response to ACh. Serotonin secretion from P‐STS cells was increased after challenge with ACh and histamine added simultaneously compared to ACh alone, suggesting that histamine increases ACh‐induced serotonin secretion from enterochromaffin cells. In conclusion, our data suggest that histamine enhances the M3R‐mediated intracellular calcium response to ACh via activation of H1R. This probably increases serotonin secretion from enterochromaffin cells and thereby affects intestinal motility in histamine intolerance, food allergies and irritable bowel syndrome.

Ca2+ influx and clearance at hyperpolarized membrane potentials modulate spontaneous and stimulated exocytosis in neuroendocrine cells

Neuroendocrine adrenal chromaffin cells release neurohormones catecholamines in response to Ca²⁺ entry via voltage-gated Ca²⁺ channels (VGCCs). Adrenal chromaffin cells also express non-voltage-gated channels, which may conduct Ca²⁺ at negative membrane potentials, whose role in regulation of exocytosis is poorly understood. We explored how modulation of Ca²⁺ influx at negative membrane potentials affects basal cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and exocytosis in metabolically intact voltage-clamped bovine adrenal chromaffin cells. We found that in these cells, Ca²⁺ entry at negative membrane potentials is balanced by Ca²⁺ extrusion by the Na⁺/Ca²⁺ exchanger and that this balance can be altered by membrane hyperpolarization or stimulation with an inflammatory hormone bradykinin. Membrane hyperpolarization or application of bradykinin augmented Ca²⁺-carrying current at negative membrane potentials, elevated basal [Ca²⁺]_i, and facilitated synchronous exocytosis evoked by the small amounts of Ca²⁺ injected into the cell via VGCCs (up to 20 pC). Exocytotic responses evoked by the injections of the larger amounts of Ca²⁺ via VGCCs (> 20 pC) were suppressed by preceding hyperpolarization. In the absence of Ca²⁺ entry via VGCCs and Ca²⁺ extrusion via the Na⁺/Ca²⁺ exchanger, membrane hyperpolarization induced a significant elevation in [Ca²⁺]_i and asynchronous exocytosis. Our results indicate that physiological interferences, such as membrane hyperpolarization and/or activation of non-voltage-gated Ca²⁺ channels, modulate basal [Ca²⁺]_i and, consequently, segregation of exocytotic vesicles and their readiness to be released spontaneously and in response to Ca²⁺ entry via VGCCs. These mechanisms may play role in homeostatic plasticity of neuronal and endocrine cells.

Histamine causes influx via T-type voltage-gated calcium channels in an enterochromaffin tumor cell line: potential therapeutic target in adverse food reactions

The P-STS human ileal neuroendocrine tumor cells, as a model for gut enterochromaffin cells, are strongly and synergistically activated by histamine plus acetylcholine (ACh), presumably via histamine 4 receptors, and weakly activated by histamine alone. Sensing these signals, enterochromaffin cells could participate in intestinal intolerance or allergic reactions to food constituents associated with elevated histamine levels. In this study we aimed to analyze the underlying molecular mechanisms. Inhibition by mepyramine and mibefradil indicated that histamine alone caused a rise in intracellular calcium concentration ([Ca²⁺]_i) via histamine 1 receptors involving T-type voltage-gated calcium channels (VGCCs). Sensitivity to histamine was enhanced by pretreatment with the inflammatory cytokine tumor necrosis factor-α (TNF-α). In accordance with the relief it offers some inflammatory bowel disease patients, otilonium bromide, a gut-impermeable inhibitor of T-type (and L-type) VGCCs and muscarinic ACh receptors, efficiently inhibited the [Ca²⁺]_i responses induced by histamine plus ACh or by histamine alone in P-STS cells. It will take clinical studies to show whether otilonium bromide has promise for the treatment of adverse food reactions. The cells did not react to the nutrient constituents glutamate, capsaicin, cinnamaldehyde, or amylase-trypsin inhibitors and the transient receptor potential channel vanilloid 4 agonist GSK-1016790A. The bacterial product butyrate evoked a rise in [Ca²⁺]_i only when added together with ACh. Lipopolysaccharide had no effect on [Ca²⁺]_i despite the presence of Toll-like receptor 4 protein. Our results indicate that inflammatory conditions with elevated levels of TNF-α might enhance histamine-induced serotonin release from intestinal neuroendocrine cells. NEW & NOTEWORTHY We show that histamine synergistically enhances the intracellular calcium response to the physiological agonist acetylcholine in human ileal enterochromaffin tumor cells. This synergistic activation and cell activation by histamine alone largely depend on T-type voltage-gated calcium channels and are inhibited by the antispasmodic otilonium bromide. The cells showed no response to wheat amylase-trypsin inhibitors, suggesting that enterochromaffin cells are not directly involved in nongluten wheat sensitivity.

Interleukin-6-mediated signaling in adrenal medullary chromaffin cells

The pro-inflammatory cytokines, tumor necrosis factor-α, and interleukin-1β/α modulate catecholamine secretion, and long-term gene regulation, in chromaffin cells of the adrenal medulla. Since interleukin-6 (IL6) also plays a key integrative role during inflammation, we have examined its ability to affect both tyrosine hydroxylase activity and adrenomedullary gene transcription in cultured bovine chromaffin cells. IL6 caused acute tyrosine/threonine phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), and serine/tyrosine phosphorylation of signal transducer and activator of transcription 3 (STAT3). Consistent with ERK1/2 activation, IL6 rapidly increased tyrosine hydroxylase phosphorylation (serine-31) and activity, as well as up-regulated genes, encoding secreted proteins including galanin, vasoactive intestinal peptide, gastrin-releasing peptide, and parathyroid hormone-like hormone. The effects of IL6 on the entire bovine chromaffin cell transcriptome were compared to those generated by G-protein-coupled receptor (GPCR) agonists (histamine and pituitary adenylate cyclase-activating polypeptide) and the cytokine receptor agonists (interferon-α and tumor necrosis factor-α). Of 90 genes up-regulated by IL6, only 16 are known targets of IL6 in the immune system. Those remaining likely represent a combination of novel IL6/STAT3 targets, ERK1/2 targets and, potentially, IL6-dependent genes activated by IL6-induced transcription factors, such as hypoxia-inducible factor 1α. Notably, genes induced by IL6 include both neuroendocrine-specific genes activated by GPCR agonists, and transcripts also activated by the cytokines. These results suggest an integrative role for IL6 in the fine-tuning of the chromaffin cell response to a wide range of physiological and paraphysiological stressors, particularly when immune and endocrine stimuli converge.

Expression and localization of the diacylglycerol kinase family and of phosphoinositide signaling molecules in adrenal gland

Adrenal glands play a central role in the secretion of steroid hormones and catecholamines. Previous studies have revealed that molecules engaged in phosphoinositide (PI) turnover are expressed in the adrenal gland, suggesting the importance of PI signaling in adrenal signal transduction. Diacylglycerol kinase (DGK) catalyzes the phosphorylation of diacylglycerol (DG), a major second messenger in the PI signaling cascade. The DGK family is expressed in distinct patterns in endocrine organs at the mRNA and protein levels. Nevertheless, little is known about the characteristics and morphological aspects of DGKs in the adrenal gland. We have performed immunohistochemical analyses to investigate the expression and localization of DGK isozymes, together with PI signaling molecules, in the adrenal gland at the protein level. Our results show that the DGK family and a set of PI signaling molecules are expressed intensely in zona glomerulosa cells and medullary chromaffin cells in the adrenal gland. In adrenal cells, DGKγ localizes to the Golgi complex, DGKε to the plasma membrane, and DGKζ to the nucleus. These findings show the distinct expression and subcellular localization of DGK isozymes and PI signaling molecules in the adrenal gland, suggesting that each DGK isozyme has a role in signal transduction in adrenal cells, especially in the zona glomerulosa and medulla.

Cav1.3 and Cav1.2 channels of adrenal chromaffin cells: emerging views on cAMP/cGMP-mediated phosphorylation and role in pacemaking

Voltage-gated Ca²⁺ channels (VGCCs) are voltage sensors that convert membrane depolarizations into Ca²⁺ signals. In the chromaffin cells of the adrenal medulla, the Ca²⁺ signals driven by VGCCs regulate catecholamine secretion, vesicle retrievals, action potential shape and firing frequency. Among the VGCC-types expressed in these cells (N-, L-, P/Q-, R- and T-types), the two L-type isoforms, Ca(v)1.2 and Ca(v)1.3, control key activities due to their particular activation-inactivation gating and high-density of expression in rodents and humans. The two isoforms are also effectively modulated by G protein-coupled receptor pathways delimited in membrane micro-domains and by the cAMP/PKA and NO/cGMP/PKG phosphorylation pathways which induce prominent Ca²⁺ current changes if opposingly regulated. The two L-type isoforms shape the action potential and directly participate to vesicle exocytosis and endocytosis. The low-threshold of activation and slow rate of inactivation of Ca(v)1.3 confer to this channel the unique property of carrying sufficient inward current at subthreshold potentials able to activate BK and SK channels which set the resting potential, the action potential shape, the cell firing mode and the degree of spike frequency adaptation during spontaneous firing or sustained depolarizations. These properties help chromaffin cells to optimally adapt when switching from normal to stress-mimicking conditions. Here, we will review past and recent findings on cAMP- and cGMP-mediated modulations of Ca(v)1.2 and Ca(v)1.3 and the role that these channels play in the control of chromaffin cell firing. This article is part of a Special Issue entitled: Calcium channels.

Equal sensitivity of Cav1.2 and Cav1.3 channels to the opposing modulations of PKA and PKG in mouse chromaffin cells

Mouse chromaffin cells (MCCs) express high densities of L-type Ca2+ channels (LTCCs), which control pacemaking activity and catecholamine secretion proportionally to their density of expression. In vivo phosphorylation of LTCCs by cAMP-PKA and cGMP–PKG, regulate LTCC gating in two opposing ways: the cAMP-PKA pathway potentiates while the cGMP–PKG cascade inhibits LTCCs. Despite this, no attempts have been made to answer three key questions related to the two Cav1 isoforms expressed in MCCs (Cav1.2 and Cav1.3): (i) how much are the two Cav1 channels basally modulated by PKA and PKG?, (ii) to what extent can Cav1.2 and Cav1.3 be further regulated by PKA or PKG activation?, and (iii) are the effects of both kinases cumulative when simultaneously active? Here, by comparing the size of L-type currents of wild-type (WT; Cav1.2+Cav1.3) and Cav1.3−/− KO (Cav1.2) MCCs, we provide new evidence that both PKA and PKG pathways affect Cav1.2 and Cav1.3 to the same extent either under basal conditions or induced stimulation. Inhibition of PKA by H89 (5 μM) reduced the L-type current in WT and KO MCCs by∼60%,while inhibition of PKG by KT 5823 (1 μM) increased by∼40% the same current in both cell types. Given that Cav1.2 and Cav1.3 carry the same quantity of Ca2+ currents, this suggests equal sensitivity of Cav1.2 and Cav1.3 to the two basal modulatory pathways. Maximal stimulation of cAMP–PKA by forskolin (100 μM) and activation of cGMP–PKG by pCPT-cGMP (1mM) uncovered a∼25% increase of L-type currents in the first case and∼65% inhibition in the second case in both WT and KO MCCs, suggesting equal sensitivity of Cav1.2 and Cav1.3 during maximal PKA or PKG stimulation. The effects of PKA and PKG were cumulative and most evident when one pathway was activated and the other was inhibited. The two extreme combinations(PKA activation–PKG inhibition vs. PKG activation-PKA inhibition) varied the size of L-type currents by one order of magnitude (from 180% to 18% of control size). Taken together our data suggest that: (i) Cav1.2 and Cav1.3 are equally sensitive to PKA and PKG action under both basal conditions and maximal stimulation, and (ii) PKA and PKG act independently on both Cav1.2 and Cav1.3, producing cumulative effects when opposingly activated. These extreme Cav1 channel modulations may occur either during high-frequency sympathetic stimulation to sustain prolonged catecholamine release (maximal L-type current) or following activation of the NO–cGMP–PKG signalling pathway (minimal L-type current) to limit the steady release of catecholamines.

Cytokine interactions with adrenal medullary chromaffin cells

It is generally accepted that a bi-directional or reciprocal interaction occurs between the immune and neuroendocrine systems, and that this relationship is important for the appropriate physiological functioning of both systems. Similarly, an imbalance in this relationship may contribute to a number of pathologies, most notably those relating to stress. The aim of this article is to consider the interaction of cytokines with the adrenal medulla, a potentially important player in this relationship. The chromaffin cells of the adrenal medulla release catecholamines and a range of biologically active peptides in response to a wide variety of stress-related signals. A growing body of evidence indicates that this stress response is influenced by, and in turn has influence upon, immune signalling. This brief review will focus primarily on the best-described adrenal medullary active cytokines, namely interferon-α, interleukin-6, interleukin-1α/β and tumour necrosis factor-α. In each case, three key issues will be addressed: the physiologically relevant source of the cytokine; the intracellular signalling events arising from activation of its receptor and finally the cellular consequences of such activation in terms of modulation of gene expression and the secretory output of the chromaffin cells.

Differential expression of the regulated catecholamine secretory pathway in different hereditary forms of pheochromocytoma

Pheochromocytomas in patients with von Hippel-Lindau (VHL) syndrome and multiple endocrine neoplasia type 2 (MEN 2) differ in the types and amounts of catecholamines produced and the resulting signs and symptoms. We hypothesized the presence of different processes of catecholamine release reflecting differential expression of components of the regulated secretory pathway among the two types of hereditary tumors. Differences in catecholamine secretion from tumors in patients with VHL syndrome (n = 47) and MEN 2 (n = 32) were examined using measurements of catecholamines in tumor tissue, urine, and plasma, the last of which was under baseline conditions in all subjects and in a subgroup of patients who received intravenous glucagon to provoke catecholamine release. Microarray and proteomics analyses, quantitative PCR, and Western blotting were used to assess expression of tumor tissue secretory pathway components. The rate constant for baseline catecholamine secretion was 20-fold higher in VHL than in MEN 2 tumors (0.359 +/- 0.094 vs. 0.018 +/- 0.009 day(-1)), but catecholamine release was responsive only to glucagon in MEN 2 tumors. Compared with tumors from MEN 2 patients, those from VHL patients were characterized by reduced expression of numerous components of the regulated secretory pathway (e.g., SNAP25, syntaxin, rabphilin 3A, annexin A7, calcium-dependent secretion activator). The mutation-dependent differences in expression of secretory pathway components indicate a more mature regulated secretory pathway in MEN 2 than VHL tumors. These data provide a unique mechanistic link to explain how variations in the molecular machinery governing exocytosis may contribute to clinical differences in the secretion of neurotransmitters or hormones and the subsequent presentation of a disease.

Adverse drug reactions in patients with phaeochromocytoma: incidence, prevention and management

The dangers of phaeochromocytomas are mainly due to the capability of these neuroendocrine tumours to secrete large quantities of vasoactive catecholamines, thereby increasing blood pressure and causing other related adverse events or complications. Phaeochromocytomas are often missed, sometimes only becoming apparent during therapeutic interventions that provoke release or interfere with the disposition of catecholamines produced by the tumours. Because phaeochromocytomas are rare, evidence contraindicating use of specific drugs is largely anecdotal or based on case reports. The heterogeneous nature of the tumours also makes adverse reactions highly variable among patients. Some drugs, such as dopamine D(2) receptor antagonists (e.g. metoclopramide, veralipride) and beta-adrenergic receptor antagonists (beta-blockers) clearly carry high potential for adverse reactions, while others such as tricyclic antidepressants seem more inconsistent in producing complications. Other drugs capable of causing adverse reactions include monoamine oxidase inhibitors, sympathomimetics (e.g. ephedrine) and certain peptide and corticosteroid hormones (e.g. corticotropin, glucagon and glucocorticoids). Risks associated with contraindicated medications are easily minimised by adoption of appropriate safeguards (e.g. adrenoceptor blockade). Without such precautions, the state of cardiovascular vulnerability makes some drugs and manipulations employed during surgical anaesthesia particularly dangerous. Problems arise most often when drugs or therapeutic procedures are employed in patients in whom the tumour is not suspected. In such cases, it is extremely important for the clinician to recognise the possibility of an underlying catecholamine-producing tumour and to take the most appropriate steps to manage and treat adverse events and clinical complications.

Retinol activates tyrosine hydroxylase acutely by increasing the phosphorylation of serine40 and then serine31 in bovine adrenal chromaffin cells

Tyrosine hydroxylase is the rate-limiting enzyme in the biosynthesis of the catecholamines. It has been reported that retinol (vitamin A) modulates tyrosine hydroxylase activity by increasing its expression through the activation of the nuclear retinoid receptors. In this study, we observed that retinol also leads to an acute activation of tyrosine hydroxylase in bovine adrenal chromaffin cells and this was shown to occur via two distinct non-genomic mechanisms. In the first mechanism, retinol induced an influx in extracellular calcium, activation of protein kinase C and serine40 phosphorylation, leading to tyrosine hydroxylase activation within 15 min. This effect then declined over time. The retinol-induced rise in intracellular calcium then led to a second slower mechanism; this involved an increase in reactive oxygen species, activation of extracellular signal-regulated kinase 1/2 and serine31 phosphorylation and the maintenance of tyrosine hydroxylase activation for up to 2 h. No effects were observed with retinoic acid. These results show that retinol activates tyrosine hydroxylase via two sequential non-genomic mechanisms, which have not previously been characterized. These mechanisms are likely to operate in vivo to facilitate the stress response, especially when vitamin supplements are taken or when retinol is used as a therapeutic agent.

Tako-Tsubo-like syndrome during anaphylactic reaction

Tako-Tsubo's syndrome (apical ballooning or broken heart syndrome) is a reversible left ventricular dysfunction due to apical asynergy that occurs typically after sudden emotional stress in a subject without coronary disease. It is characterized by acute onset of chest pain or dyspnoea or both and is associated with electrocardiographic changes such as ST segment elevation and/or T wave inversion. Myocardial biomarkers may be normal or slightly elevated. Anaphylaxis is a severe, life-threatening, generalized hypersensitivity reaction, most often starting with urticaria and/or angioedema, that may involve cardiovascular and respiratory systems. Cardiovascular symptoms, including hypotension, cardiac arrhythmia and chest pain, are presumably linked to cardiac mast cell mediator release. We describe the case of a young woman who experienced a profound reversible cardiomyopathy with typical features of Tako-Tsubo's syndrome during an anaphylactic reaction.

Potentiation of exocytosis by phospholipase C-coupled G-protein-coupled receptors requires the priming protein Munc13-1

The vesicle priming protein Munc13-1 is regulated by diacylglycerol (DAG) and is therefore hypothesized to play a role in the control of neurotransmitter release by phospholipase C (PLC)-coupled receptors. We combined voltage-clamp recordings of voltage-gated Ca2+ channels (VGCCs) and high-resolution capacitance measurements to investigate the mechanism of receptor-mediated modulation of exocytosis in bovine chromaffin cells. Activation of endogenous H1 G(q)-protein-coupled receptors (G(q)PCRs) by histamine potentiated stimulus-coupled secretion despite concurrently inhibiting Ca2+ influx through VGCCs. Histamine increased the size of the readily releasable pool of vesicles and in particular a subpool of fusion-competent vesicles localized in close proximity to VGCCs. Pharmacological characterization showed that potentiation of exocytosis depended on the activation of PLC but not protein kinase C. Overexpression of wild-type Munc13-1 by adenoviral infection had no effect on histamine-induced potentiation per se, whereas DAG-insensitive Munc13-1(H567K) completely abolished it. This is the first endogenous mammalian G(q)PCR signaling pathway identified that engages Munc13-1 to increase stimulus-coupled secretion by recruiting vesicles to the immediately releasable pool. G(q)PCRs are therefore able to control exocytosis at the level of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex formation to produce rapid, short-term potentiation of the secretory output of neurons and endocrine cells.

The hop cassette of the PAC1 receptor confers coupling to Ca2+ elevation required for pituitary adenylate cyclase-activating polypeptide-evoked neurosecretion

We have identified the single PAC1 receptor variant responsible for Ca2+ mobilization from intracellular stores and influx through voltage-gated Ca2+ channels in bovine chromaffin cells and the domain of this receptor variant that confers coupling to [Ca2+]i elevation. This receptor (bPAC1hop) contains a 28-amino acid "hop" insertion in the third intracellular loop, with a full-length 171-amino acid N terminus. Expression of the bPAC1hop receptor in NG108-15 cells, which lack endogenous PAC1 receptors, reconstituted high affinity PACAP binding and PACAP-dependent elevation of both cAMP and intracellular Ca2+ concentrations ([Ca2+]i). Removal of the hop domain and expression of this receptor (bPAC1null) in NG108-15 cells reconstituted high affinity PACAP binding and PACAP-dependent cAMP generation but without a corresponding [Ca2+]i elevation. PC12-G cells express sufficient levels of PAC1 receptors to provide PACAP-saturable coupling to adenylate cyclase and to drive PACAP-dependent differentiation but do not express PAC1 receptors at levels found in postmitotic neuronal and endocrine cells and do not support PACAP-mediated neurosecretion. Expression of bPAC1hop, but not bPAC1(null), at levels comparable with those of bPAC1hop in bovine chromaffin cells resulted in acquisition by PC12-G cells of PACAP-dependent [Ca2+]i increase and extracellular Ca2+ influx. In addition, PC12-G cells expressing bPAC1hop acquired the ability to release [3H]norepinephrine in a Ca2+ influx-dependent manner in response to PACAP. Expression of PACAP receptors in neuroendocrine rather than nonneuroendocrine cells reveals key differences between PAC1hop and PAC1null coupling, indicating an important and previously unrecognized role of the hop cassette in PAC1-mediated Ca2+ signaling in neuroendocrine cells.

Arachidonic acid potentiates exocytosis and allows neuronal SNARE complex to interact with Munc18a

Neuronal communication relies on the fusion of neurotransmitter-containing vesicles with the neuronal plasma membrane. Recent genetic studies have highlighted the critical role played by polyunsaturated fatty acids in neurotransmission, however, there is little information available about which fatty acids act on exocytosis and, more importantly, by what mechanism. We have used permeabilized chromaffin cells to screen various fatty acids of the n-3 and n-6 series for their acute effects on exocytosis. We have demonstrated that an n-6 series polyunsaturated fatty acid, arachidonic acid, potentiates secretion from intact neurosecretory cells regardless of the secretagogue used. We have shown that arachidonic acid dose dependently increases soluble NSF attachment protein receptor complex formation in chromaffin cells and bovine cortical brain extracts and that a non-hydrolysable analogue of arachidonic acid causes a similar increase in SNARE complex formation. This prompted us to examine the effect of arachidonic acid on SNARE protein interactions with Munc18a, a protein known to prevent Syntaxin1a engagement into the SNARE complex in vitro. In the presence of arachidonic acid, we show that Munc18a can interact with the neuronal SNARE complex in a dose-dependent manner. We further demonstrate that arachidonic acid directly interacts with Syntaxin1a.

Phospholipase C isozymes are differentially distributed in the rat adrenal medulla

Immunohistochemistry has been used to examine the distribution of selected phospholipase C (PLC) isozymes within the adrenal medulla of the rat. PLCbeta isozymes were expressed at moderate levels in the chromaffin cells but more strongly in association with ganglion cell clusters. PLCbeta2 and PLCbeta3 staining of clusters did not overlap suggesting selective PLC isozyme expression in two distinct ganglionic types. The distribution of PLCbeta4 immunoreactivity was very similar to PLCbeta3 with the strongest staining observed in the same cell clusters. Antibodies to PLCbeta1 labelled multiple bands on Western blots and were not therefore used for immunohistochemistry. The chromaffin cells were also immunoreactive for PLCgamma1, although the strongest staining with this antibody was seen in cells surrounding large sinus vessels. PLCdelta1 and PLCdelta2 had quite distinct distributions, with the former selectively localized to an endothelial cell population surrounding the chromaffin cells. This observation was supported by experiments on isolated bovine adrenal medullary cells where PLCdelta1 expression was lost when the cell preparation was enriched for chromaffin cells. Antibodies to PLCdelta2 labelled a network of nerve fibres throughout the medulla and clusters of ganglion cells located primarily at the medullary-cortical boundary. PLCdelta2 immunoreactivity was also present in nerve fibres within the adrenal capsule where it appeared to be co-localized with PLCbeta4 staining.

Linopirdine Modulates Calcium Signaling and Stimulus-Secretion Coupling in Adrenal Chromaffin Cells by Targeting M-Type K+Channels and Nicotinic Acetylcholine Receptors

Adrenal chromaffin cells synthesize and release catecholamines and several other transmitters that play important physiological roles in the coordinated response to stress or danger. The main trigger for secretion is acetylcholine (ACh) released from splanchnic nerve terminals that activates nicotinic ACh receptors (nAChRs) on the chromaffin cells, causing membrane depolarization and Ca2+ entry primarily through voltage-gated Ca2+ channels (Ca-channels). G protein-coupled receptors (GPCRs) can also trigger secretion, and it has been suggested that closure of M-type K+ channels might contribute to this process. However, GPCRs have multiple effects on calcium signaling and secretion, including release of intracellular Ca2+ stores, activation of second messenger pathways and kinases, and Ca2+ entry through store/receptor-operated channels. Hence, the effects of M-channel closure on [Ca2+]i signaling and transmitter release remain unclear. We have investigated the effects of linopirdine, a relatively selective blocker of M-channels, on stimulus-secretion coupling in chromaffin cells. Linopirdine produced a small increase in [Ca2+]i in approximately 63% of cells because of influx through Ca-channels. However, this was not sufficient to promote catecholamine release. We also show that linopirdine reduced cholinergic-stimulated increases in [Ca2+]i and secretion, primarily through potent block of nAChRs and a subtle effect on Ca2+ entry via Ca-channels. Hence, our data support the hypothesis that M-channels help control the excitability of chromaffin cells, but additional pathways need to be recruited by GPCRs to trigger catecholamine release. Furthermore, linopirdine potently targets nAChRs to modulate stimulus-secretion coupling in adrenal chromaffin cells.

Bimodal regulation of the human H1 histamine receptor by G protein-coupled receptor kinase 2

The H1 histamine receptor (H1HR) is a member of the G protein-coupled receptor superfamily and regulates numerous cellular functions through its activation of the G(q/11) subfamily of heterotrimeric G proteins. Although the H1HR has been shown to undergo desensitization in multiple cell types, the mechanisms underlying the regulation of H1HR signaling are poorly defined. To address this issue, we examined the effects of wild type and mutant G protein-coupled receptor kinases (GRKs) on the phosphorylation and signaling of human H1HR in HEK293 cells. Overexpression of GRK2 promoted H1HR phosphorylation in intact HEK293 cells and completely inhibited inositol phosphate production stimulated by H1HR, whereas GRK5 and GRK6 had lesser effects on H1HR phosphorylation and signaling. Interestingly, catalytically inactive GRK2 (GRK2-K220R) also significantly attenuated H1HR-mediated inositol phosphate production, as did an N-terminal fragment of GRK2 previously characterized as a regulator of G protein signaling (RGS) protein for Galpha(q/11). Disruption of this RGS function in holo-GRK2 by mutation (GRK2-D110A) partially reversed the quenching effect of GRK2, whereas deletion of both the kinase activity and RGS function (GRK2-D110A/K220R) effectively relieved the inhibition of inositol phosphate generation. To evaluate the role of endogenous GRKs on H1HR regulation, we used small interfering RNAs to selectively target GRK2 and GRK5, two of the primary GRKs expressed in HEK293 cells. A GRK2-specific small interfering RNA effectively reduced GRK2 expression and resulted in a significant increase in histamine-promoted calcium flux. In contrast, knockdown of GRK5 expression was without effect on H1HR signaling. These findings demonstrate that GRK2 is the principal kinase mediating H1 histamine receptor desensitization in HEK293 cells and suggest that rapid termination of H1HR signaling is mediated by both the kinase activity and RGS function of GRK2.